Light-sensitive silver halide photographic materials, in particular, light-sensitive silver halide color photographic materials have a high sensitivity and give an excellent gradation. Hence, they are nowadays in a very wide use.
However, light-sensitive silver halide color photographic materials are photographically processed by what is called wet processing, which has been involved in the disadvantages that it takes much time and labor for the preparation of processing solutions, pollutes surroundings, produces waste fluids containing various chemicals, requires darkrooms and takes a long time until a first print is obtained after start of operation. To offset such disadvantages and make the most of the light-sensitive silver halide color photographic materials, it has been widely established that skilled engineers handle the processing from the development of color negatives up to the preparation of color prints in a centralized system.
Meanwhile, although there are no fundamental changes in the wet processing, a series of improvements in machinery such as printers and automatic processors, improvements in photographic processing solutions and improvements in light-sensitive silver halide color photographic materials and forms for their packaging have recently brought about a rapid spread of what is called minilabs that can continuously operate the processing from the development of color negatives up to the preparation of color prints at a small place such as a storefront of photograph shops.
In particular, employment of silver halide emulsions containing silver chloride in a high concentration has made processing time remarkably shorter. However, the silver halide emulsions containing silver chloride in a high concentration are known to be inferior in the stability of performances during storage, and an advance has been made on techniques in which, e.g., a specific nitrogen-containing heterocyclic mercapto compound is used. None of conventional techniques, however, have achieved a satisfactory improvement, where, in particular, a decrease in sensitivity in an environment of room temperature and high humidity has been questioned.
It is hitherto well known to add a spectral sensitizer to a silver halide photographic emulsion in an undissolved state.
For example, Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) No. 135437/1990 discloses that no flocculates can be caused and coating difficulties can be decreased by a technique in which a photographic additive having a good dispersibility in water is added in water, followed by dispersion using a dispersion machine, and thereafter a photographic additive substantially insoluble in the resulting dispersion and having a high solubility in a surface active agent is added together with a surface active agent having a sulfonic acid group or sulfate group as a hydrophilic group, to give a dispersion, which is then added to a silver halide emulsion. Similarly, Japanese Patent O.P.I. Publications No. 135438/990, No. 23331/1990 and No. 23332/1990 disclose methods in which a dispersion of a photographic additive is obtained using a surface active agent. None of them, however, have any specific disclosure as to photographic performances.
Japanese Patent O.P.I. Publication No. 23330/1990 also discloses that coating difficulties can be decreased by a method in which a water-insoluble photographic additive is heated and dissolved using an organic solvent together with at least one readily water-soluble photographic additive until they completely dissolve, followed by evaporation of the organic solvent from the resulting solution to give a flaky concentrate, thereafter the flaky concentrate is dispersed in water, and then the resulting dispersion is added to a silver halide emulsion. This publication, however, is also silent as to photographic performances.
Japanese Patent O.P.I. Publication No. 196749/1985 discloses that an increase in the rate of development and an improvement in the prevention of reciprocal law failure can be achieved by adding a spectral sensitizer without use of any organic solvent, before the formation of silver halide grains is completed. Japanese Patent O.P.I. Publication No. 105141/1983 also discloses that the desired sensitivity can be achieved and no coating difficulties can be made to occur by the use of a dispersion obtained by mechanically pulverizing a spectral sensitizer into fine particles of 1 .mu.m or smaller in an aqueous system to effect dispersion, under conditions of pH 6 to 8 and 60.degree. to 80.degree. C. In Examples thereof, it is disclosed to use a dispersion prepared by dispersing a spectral sensitizer using a high-speed stirrer. However, the silver halide photographic emulsion used therein is a pure silver bromide or silver iodobromide emulsion or a silver chlorobromide emulsion containing silver bromide in a high content, and there is neither disclosure nor suggestion as to performances questioned when a solid dispersion of a spectral sensitizer is used in a silver chlorobromide emulsion containing silver chloride in a high content or a pure silver chloride emulsion. Similar techniques are also disclosed in Japanese Patent O.P.I. Publications No. 125631/1992, No. 125632/1992, No. 125634/1992, etc. These publications also have neither disclosure nor suggestion as to the disadvantage originating from the silver chlorobromide emulsion containing silver chloride in a high content or the pure silver chloride emulsion and as to the performances questioned when a solid dispersion of a spectral sensitizer is used in these emulsions.
Japanese Patent O.P.I. Publications No. 80119/1975 and No. 80826/1975 disclose that a high sensitivity can be achieved by a method in which a substantially water-insoluble photographic additive is dissolved in an acid having pKa not more than 5, the resulting solution is dispersed in a water-based solution, and the resulting dispersion is added to a silver halide photographic emulsion. However, the halogen composition of the silver halide photographic emulsion specifically described therein is silver chlorobromide mainly composed of silver bromide, and there is neither disclosure nor suggestion as to the performances questioned when a solid dispersion of a spectral sensitizer is used in the silver chlorobromide emulsion containing silver chloride in a high content or the pure silver chloride emulsion. In addition, the weight ratio of silver halide (in terms of silver) to gelatin (Ag/Gel) at the time of spectral sensitization is 1 or less, and there is no disclosure as to the effect of the present invention obtained when a chloride-rich silver halide emulsion is spectrally sensitized under a specific ratio of silver halide to gelatin.